The imaging apparatus is widely used in identification processes, e.g., moving body controllers controlling moving bodies (moving apparatuses) such as vehicles, vessels, aircrafts and industrial robots and information providers providing useful information to drivers of the moving bodies. Specifically, the imaging apparatus used in driver support system such as ACC (Adaptive Cruise Control) reducing drive load of vehicle drivers is known. The vehicle drive support system includes various functions such as an automatic braking system and a warning system preventing a vehicle from crashing into obstacles and reducing an impact of crashing, an automatic velocity controller for keeping a distance with a vehicle ahead, and an assisting system to prevent a vehicle from veering out of its lane. For these purposes, it is essential to obtain various environmental information around a vehicle as accurately as possible by analyzing images around the vehicle. The environmental information around a vehicle includes positional information of various objects, e.g., obstacles present around the vehicle, other vehicles, traffic lanes such as white lines and Botts Dots, road surface compositions such as manhole covers, road edge structures such as guard rails; road surface wet-dry information; and sunlight information informing sunny or shady, etc.
A typical imaging apparatus detects light intensity (luminance information) and forms an image according to the luminance information. However, an imaging apparatus capable of forming an image (polarization information image) reflecting polarization information for detection (sensing) of a shape, a material, a surface status, etc. of an object present in an imaging area has been attracting attention recently. The imaging apparatus uses reflection light (specular reflection light or diffuse reflection light) from an object irradiated with a particularly polarized light or non-polarized natural light, which is various partially-polarized light according to geometric elements such as a direction of the surface of the object and an imaging position relative thereto, and the surface material thereof. The imaging apparatus is capable of obtaining a two-dimensional distribution of plural polarization components having polarization directions different from each other included in the reflection light from the object in the imaging area. Comparing with the differences among the polarization components included in the light from the object in the imaging area, the positional information of the object, the road surface wet-dry information, the sunlight information, etc. can more accurately be obtained.
Japanese published unexamined application No. 11-175702 discloses an imaging apparatus parallely locating an imaging element imaging through a polarization filter transmitting only a perpendicular polarization component and an imaging element imaging through a polarization filter transmitting only a horizontal polarization component. In the imaging apparatus, the former imaging element produces an image signal of a perpendicular polarization image representing a two-dimensional intensity distribution of the perpendicular polarization component included in the reflection light from the object in the imaging area, and the latter imaging element produces an image signal of a horizontal polarization image representing a two-dimensional intensity distribution of the horizontal polarization component included in the reflection light from the object in the imaging area. The imaging apparatus corrects a parallax positional gap of the image signals of the perpendicular polarization image and the horizontal polarization image, and determines a ratio of a perpendicular polarization intensity to a horizontal polarization intensity per a pixel to obtain a polarization ratio image (polarization information image) having a pixel value which is the polarization ratio.
Analyzing the polarization ratio image imaged by the imaging apparatus disclosed in Japanese published unexamined application No. 11-175702 enables it to obtain the above-mentioned various information such as positional information of the object. Not only the polarization ratio image imaged, but also the polarization information image having a pixel value based on an index value representing differences in sizes of polarization components included in light from each point in the imaging area, e.g., a difference value among these polarization components and a ratio thereof to a total value of these polarization components does as well.
An inaccurate index value is determined when even one of the polarization components used for determining the index value received by a light receiving element has a saturated amount of light received which is a maximum value of a light quantity detection range. When the polarization information image including the inaccurate index values is analyzed, the analysis results have errors and it is difficult to obtain accuracy of the above-mentioned various information. In order to avoid this, imaging at sufficiently low light exposure such that the light receiving element does not have a saturated value of the amount of light received is recommended. However, any of the polarization components has a low amount of light received, and the index value representing differences in sizes of the polarization components has a narrow range. As a result, the polarization information image having a pixel value based on an index value has a low contrast, resulting in inaccurate analysis thereof.
Because of these reasons, a need exist for an imaging apparatus capable of accurately analyzing a polarization information image having a pixel value based on an index value representing differences in sizes of polarized components included in light from each point in an imaging area.